The project addresses basic reflex functions (autonomic and somatic) mediated by the brain's defense motive system. Three sets of experiments with human participants are proposed that have the following broad aims: (1) to determine autonomic and somatic correlates of the primitive defense reaction occasioned by "looming" stimuli-- signaling imminent collision and evoking a reflex similar to startle-- and furthermore, to define brain structures critical in processing "looming" stimuli and in mediating the defense response; (2) to assess the effects of repeated aversive stimulation and elucidate conditions that promote the habituation or sensitization of psychophysiological indices of unpleasant emotion, and to asses related changes in brain function (3) To determine patterns of change (defense reflex physiology and brain activation patterns) on avoidance gradients that define increasing imminence of threat. The research examines what can be considered fundamental features of aversive stimulus processing--reflex reactions that are similar in animals and humans. In pursuing these aims, research begins in the psychophysiological laboratory with measurement of autonomic and somatic reflex responses. High-density EEG in also recoded in this context to determine co-incident electro-cortical events and locate dipole sources. Subsequent experiments track BOLD activity in the MR scanner, to better determine the brain structures and circuits that mediate emotion's reflexes.